Electrostatic printing



Nov. 24, 1959 M. L. SUGARMAN, JR

ELECTROSTATIC PRINTING Filed May 17, 1955 SECOND 0VLOP SECOND CWHAG'EEXPOSE INVENTOR. Jana Z. duummzx; JR.

l7- TOKA/E Y United States Patent Q ELECTROSTATIC PRINTING Meyer L.Sugarman, Jr., Princeton, NJ., assignor to Radio Corporation of America,a corporation of Delaware Application May "17, 1955, Serial No. 508,990

1 3 Claims. (Cl. 96-1) This invention relates generally to improvedmethods and apparatus for producing latent electrostatic charge patternsand for electrostatic printing.

An electrostatic printing process is that type of process for producinga visible record, reproduction or copy which includes as an intermediatestep, converting a light image or electric signal to a latentelectrostatic charge pattern on an electrically-insulating substrate.The process may also include converting the charge pattern into avisible image which may be a substantially faithful reproduction of theoriginal except that it may be different in size, color or contrastvalue.

An object of the invention is to provide improved methods and means ofelectrostatic printing.

Another object is to provide improved methods and apparatus forproducing latent electrostatic images and electrostatic charge patterns.

Another object is to provide improved methods and means for producinglatent electrostatic images of a desired strength and polarity from anoriginal latent electrostatic image.

A further object is to provide improved methods and means for producinga reverse electrostatic image of an existing physical or electrostaticimage.

In general, the electrostatic printing processes and apparatus of theinvention comprise steps of and means for producing a powder image in adesired configuration upon a surface of a material having charge storageproperties and then exposing said surface to a discharge ofelectrically-charged particles whereby areas of said surface not coveredby said powder image become electrostatically charged. The originalpowder image may be produced, for example, by depositing powder througha stencil or by an electrophotographic process. The deposited powderimage and its background are exposed, for example, to a corona dischargeof the desired polarity until an electrostatic image of the desiredstrength is produced in the unmasked areas of the supporting surface.The electrostatic image produced thereby may then be used for anydesired purpose, for example, it may be developed to a visible image.The practice of the invention permits the production of electrostaticimages of a desired strength and polarity utilizing as a masking mediuma powder image laid down by any method and from other latentelectrostatic images.

The foregoing objects and other advantages will be described in greaterdetail in the following description when read in conjunction with theaccompanying drawings in which:

Figure 1(a), (b), (c), (d) and (e) are partially-sectional,partially-schematic views of a first apparatus illustrating the stepsfor carrying out the improved processes of the invention, and

Figure 2 is a partially-sectional, partially-schematic view of a secondapparatus for carrying out the improved processes of the invention.

Similar reference characters are applied to similar ele ments throughoutthe drawing.

2,914,403 Patented Nov. 24, 1959 Referring to Figure 1, a typicalembodiment of the has charge storage properties. Such a sheet maycomprise a single electrically-insulating material such as cellophane,glass, tetrafiuoroethylene resin or mica. Alternatively, the sheet maycomprise a relatively conducting material having a coating thereon of amaterial which is electrically-insulating, such as aluminum foil havinga coating of polystyrene or copper having a coating of a celluloseacetate resin. sheet 19 comprises a sheet of tetrafluoroethylene resin.

A powder image 23 is produced upon the electricallyinsulating surface ofthe sheet 19. For purposes of illustration, such a powder image may beproduced by placing a stencil upon the electrically-insulating surface21, dusting a powder thereon and then removing the stencil with theexcess powder. Such a powder image 23 may also be produced bydischarging electricity upon predetermined portions of the surface 21 toform electrostatically-charged areas of a desired configuration, andthen applying thereto an electroscopic powder in the manner forproducing Lichtenburg figures.

The electrically-insulating surface 21 with the powder image 23 thereonis then exposed to a uniform discharge of electrically-chargedparticles. pose, a unit comprising a plurality of three mil coronadischarge wires 27 spaced about 0.5 inch from each other and from theelectrically-insulating surface 21 is connected to a voltage source V(not shown) of the desired polarity and voltage. As an example, anegative voltage of 6400 volts is applied to the wires 27 producing acorona discharge therefrom which is attracted to a grounded backingplate 25 upon which the sheet 19 rests. The wires 27 are moved acrossthe surface 21 producing a substantially uniform electrostatic charge onthe surface 21 of the sheet 19. It may be desirable for the wires 27 tomake several passes over the surface 21 although one slow pass usuallywill suffice. Electricallycharged' particles may also be produced fromradioactive sources of a desired type or from thermionic sources.

The corona discharge from the wires 27 produces a substantially uniformelectrostatic charge across the surface 21 hearing the powder image 23.A portion of the charge resides upon the powder image 23 and a portionresides upon the electrically, insulating surface 21 not covered by thepowder image 23. The powder image 23 therefore masks theelectrically-insulating surface 21 beneath it from theelectrostatically-charged particles emanating from the corona dischargewires 27.

The sheet 19 is preferably removed from the backing plate 25, althoughit may be permitted to rest thereon. A nozzle 29 directs a stream of airupon the surface 21 to blow away the powder image 23 leaving only theelectrically-insulating surface 21 having electrostaticallycharged areasthereon in the areas not masked by the powder image 23. A perforatedcontainer 31 may then be used to deposit a developer powder 23 bysprinkling it upon the electrically-insulating surface 21 where itdeposits upon the charged areas to develop the latent electrostaticimage to a visible image 35.

In another embodiment of the invention shown in Figure 2, a papersubstrate 53 having a photoconducting coating 51 on one surface thereofpasses between and over pulleys 47 and 49. A series of stations forproducing visible images by an electrostatic printing process aredisposed along the path of said sheet between said pulleys 47 and 49.The photoconducting coating 51 is preferably a photoconductive zincoxide dispersed in a silicone resin. A preferred photoconducting coatingmay be prepared as follows: a mixture of 65 grams of a 60% solution of asilicone resin dispersed in xylene (a com- For purposes of illustration,

For this purmercially available product is GE SR-82 marketed by TheGeneral Electric Company, Silicone Products Division, Waterford, NewYork), 85 grams of toluene and 100 grams of zinc oxide having a highvalue of surface photooonductivity are ball milled together to a smoothuniform consistency. The mixture is applied to one surface of a paperweb 53 by any standard coating technique, for example, flowing,spraying, dipping, whirling or brushing on, and then dried. Upon dryingthe coated paper is ready for use. Other photoconducting coatings suchas an evaporated selenium or sublimed anthracene or lead iodidedispersed in cellulose acetate may be used. Similarly, other substratessuch as brass, Zinc, treated paper or aluminum may be used.

The photoconducting coating 51 is provided with a uniformelectrostaticcharge on the surface thereof. For this purpose, a plurality of threemil corona discharge wires, disposed together and spaced 0.5 inch fromeach other and from the photoconducting coating 51, are connected to avoltage source V (not shown) of the desired polarity and voltage. As anexample, a negative voltage of 6400 volts produces a corona dischargewhich will deposit a negative electrostatic charge upon the surface ofthe photoconducting coating 51. As the photoconducting coating 51 passesbeneath the wires 57, a corona discharge issuing therefrom is attractedto a grounded backing plate 55 placed behind the paper substrate 53 anddeposits upon the surface of the coating 51. A positive electrostaticcharge may be produced on the coating 51 by applying a positive voltageto the wires 57.

The electrostatically-charged photoconductive coating 51 now passes to astation where it is exposed to an incident electromagnetic radiationimage, for example, a light image from a projector 59. Theelectromagnetic radiation may be derived by any of the methods wellknown in the photographic art, for example, by projection from aphotographic transparency. The electromagnetic radiation image may beinfrared, visible, ultraviolet or X-rays for example and should be atype of radiation to which the photoconducting coating 51 is sensitive.Upon exposure to the electromagnetic radiation image, the illuminatedareas of the photoconducting coating 23 discharge the charge stored onthe surface thereof leaving the non-illuminated areas charged, thusproducing a first latent electrostatic image substantially correspondingto the electromagnetic radiation image.

The first latent electrostatic image is developed to a visible imageaccording to any of the methods well known in the electrostatic printingart. For example, a grounded rotary pole piece 61 of a magneticstructure having spaced parallel inclined elliptical discs 63 along itslength faces the photoconductive coating 51 and a fixed magnetic polepiece 65 faces the substrate side of the sheet opposite the rotarymagnetic pole piece 63. A magnetic field is maintained therebetweenthrough an enclosed magnetic field path and across the gap therebetweenthrough which the sheet passes. A trough 67 holds a quantity ofdeveloper mix which comprises developer powder particles and magneticcarrier particles in contact with the discs 63 such that, if the discsare rotated in a clockwise position, as viewed in Figure 2, a quantity69 of developer mix is carried upwardly and swept across the surface ofthe photoconducting coating 23, depositing developer powder particles inselected areas thereby producing a developed powder image 23 on thesurface of the photoconductive coating 51 corresponding to the firstlatent electrostatic image.

A preferred-carrier material for the developer mix consists ofalcoholized iron, that is, iron particles free from grease and otherimpurities soluble in alcohol. These iron particles are preferablyrelatively small in size, being in their largest dimensions from about0.003 inch minimum to 0.008 'inch maximum. Satisfactory results are alsoobtained using carriers consisting of iron particles of somewhat widerranges up to about 0.001 to 0.020 inch.

A preferred developer powder may be prepared as fol lows: a mixturecomprising 200 grams of 200 mesh piccolastic resin 4358 (an elasticthermoplastic resin composed of polymers of styrene, substituted styreneand its homologs) marketed by the Pennsylvania Industrial Company,Clairton, Pa., and 12 grams of carbon black G marketed by the Eimer andAmend Company, New York, N.Y., are throughly mixed in a stainless steelbeaker at about 200 C. The mixing and heating should be done in as shorta time as possible. The melt is poured upon a brass tray and allowed tocool and harden. The hardened mix is then broken up and ball milled forabout 20 hours. The powder is screened through a 200 mesh screen and isthen ready for use as a developer powder. This powder takes on apositive electrostatic charge when mixed with iron powder. It thereforewill develop the negatively charged areas of an electrostatic image.About 2 to 10 grams, preferably 4 grams, of the developer powder andabout grams of the magnetic carrier material are blended together givingthe preferred developer Other ratios of developer powder to magneticcarrier material may be used.

The photoconducting coating 51 now passes to a station where it isexposed to blanket illumination from a source 71 in a shield 73 for thepurpose of discharging the electrostatic charges on the surface of thephotoconducting coating 5-1. The electromagnetic radiation for thispurpose is most conveniently derived from an incandescent or afluorescent lamp and is of the type to which the photoconducting coating51 is sensitive.

The photoconducting coating 51 now passes to a station where thephotoconducting coating with the developed powder image is exposed to auniform discharge of electrostatically-charged particles in the samemanner as heretofore described. For this purpose a grounded backingplate '25 placed behind the paper substrate 53 and corona dischargewires 27 spaced from one another and from the photoconducting coating byabout 0.5 inch are connected to a voltage source V (not shown). Anegative voltage of about 6400 volts, for example, is applied to thecorona discharge wires 27 causing a uniform discharge therefrom. Thedischarge from the wires 27 is attracted to the grounded backing plate25, producing a negative charge upon the powder image 23 and the areasof the photoconducting coating not covered by the powder image 23passing below. The voltage and the speed of advance of thephotoconducting coating 51 is adjusted so that the desired magnitude ofcharge may be formed on the coating 51. A positive charge may beproduced by applying a positive voltage to the wires 27. Thus, the firstlatent electrostatic image is utilized by the invention to produce asecond latent electrostatic image having areas of a desired polarity andmagnitude of charge. In the example of Figure 2, the negativelychargedareas of the first latent electrostatic image are those areas covered bythe powder image 23. The negatively-charged areas of the second latentelectrostatic image correspond to the areas not covered by the powderimage 23. Thus, by the method of the invention, the latent electrostaticimage is reversed, that is, negative areas of a first latentelectrostatic image are now positively-charged and positively chargedareas of the first latent electrostatic images are nownegatively-charged. The foregoing example describes a process wherein anegative voltage is applied to both sets of corona discharge wires 57and 27. Similar results are obtained where a positive voltage is appliedto both sets of corona discharge wires 57 and 27. Where a positivevoltage is applied, it is preferred to utilize developing substancesthat have a greater affinity for positively-charged areas of a latentelectrostatic image.

Where a negative voltage is applied to the first corona wires 57 and apositive voltage is applied to the second corona wires 27, the latentelectrostatic image is not reversed. However, a econd latentelectrostatic image substantially identical to the first latentelectrostatic images is produced except that the electric fieldsemanating therefrom may be either stronger or weaker than the fieldsemanating from the first electrostatic image. The process of theinvcntion therefore, may be used to strengthen or weaken a latentelectrostatic image.

The same condition holds where a positive voltage is applied to thefirst corona wires 57 and a negative voltage is applied to the secondcorona wires 27. Again,

the latent electrostatic image is not reversed, although it may bestrengthened or weakened as desired.

The photoconductive coating 51 hearing the first developed image 23 andthe second latent electrostatic image now passes to a station whichremoves the first developed image and develops the second latentelectrostatic image. The developing station has a grounded rotary polepiece 75 of a magnetic structure with spaced parallel inclinedelliptical discs 77 along its length facing the photoconductive layer 51and a fixed magnetic pole piece 79 facing the paper substrate 53. Amagnetic field is maintained between the fixed magnetic pole piece 79and the elliptical discs 77 through an enclosed magnetic field path andacross the gap between the discs 77 and the fixed pole piece 79. Atrough 81 holds a quantity of magnetic developer mix in contact with thediscs 77 such that, as the discs are rotated in a clockwise direction asviewed in Figure 2, a quantity 83 of the developer mix is carriedupwardly and swept across the surface of the photoconductive coating 51wiping away the first developed image 23 and depositing developer powderparticles in selected areas thereby producing a second developed imageupon the surface of the photoconductive coating 51 corresponding to thesecond latent electrostatic image.

The photoconductive coating 51 now passes to a station where the seconddeveloped image is fixed to the photoconductive coating 51. For thispurpose an electric resistance element 87 is connected to a voltagesource V-V (not shown) which radiates heat sufficient to fuse the powderimage 85 to the photoconductive coating 51 fixing the developed imagethereto. Other methods of fixing such as spraying with an adhesive orsolvent vapors and/or pressure may be used.

There have been described improved methods and means for producinglatent electrostatic charge patterns and for electrostatic printing.According to the inventien one may produce latent electrostatic imagesof a desired strength and polarity from powder images disposed on anelectrically-insulating substrate and formed by physical means orelectrophotographic means.

What is claimed is:

1. A method of electrostatic printing comprising the steps of producinga first latent electrostatic image on a photoconducting insulatingsurface, developing said first latent electrostatic image with afinely-divided developer powder thereby forming a first visible powderimage on said surface, uniformly exposing said surface bearing saidfirst visible powder image to light to which said surface is sensiti eto s bstantailly discharge the electrostatic charges remaining on saidsurface after producing said first latent electrostatic image whileleaving said first visible powder image substantially undisturbed andadhering to said surface, exposing said surface bearing said firstvisible powder image thereon to a corona discharge to produce a secondlatent electrostatic image thereon in reverse configuration with respectto said first latent electrostatic image but having the same polarity ofcharge as said first latent electrostatic image, removing said firstvisible powder image leaving said second latent electrostatic imagesubstantially undisturbed, and developing said second latentelectrostatic image with a finely-divided developer powder to produce asecond visible powder image in reverse configuration with respect tosaid first visible powder image.

2. A method of electrostatic printing comprising the steps of exposingphotoconductive insulating surface to a corona dischargeto produce asubstantially uniform electrostatic charge across said surface, exposingsaid surface to a light image of a type to which said surface issensitive to produce thereon a first latent electrostatic image,developing said first latent electrostatic image with a finely-divideddeveloper powder thereby forming a loosely held first visible powderimage on said surface, uniformly exposing said surface bearing saidfirst visible powder image to light of a type to which said surface issensitive to substantially discharge the electrostatic charges remainingon said surface after exposing said surface to said light image whileleaving said first powder image substantially undisturbed and adheringto said surface, exposing said surface to a corona discharge to producethereon a second latent electrostatic image in reverse configurationwith respect to said first latent electrostatic image but having thesame polarity of charge as said first latent electrostatic image,removing said first visible powder image leaving said second latentelectrostatic image substantially undisturbed and developing said secondlatent electrostatic image with a finely-divided developer powder toproduce a second visible powder image in reverse configuration withrespect to said first visible powder image.

3. Method of claim 1 wherein removal of said first visible powder imageis accomplished concurrently with the production of said second powderimage by contacting across said surface a mixture of magnetic carrierparticles and developer powder particles held in a loose coherent masswith a magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS2,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 19442,573,881 Walkup et al Nov. 6, 1951 2,647,464 Ebert Aug. 4, 19532,681,473 Carlson June 22, 1954 2,74l,959 Rheinfrank Apr. 17, 19562,752,833 Jacob July 3, 1956 2,756,676 Steinhilper July 3, 19562,808,328 Jacob Oct. 1, 1957 2,817,598 Hayford Dec. 24, 1957 2,833,648Walkup May 6, 1953 FOREIGN PATENTS 154,222 Australia Nov. 18, 1953

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF PRODUCINGA FIRST LATENT ELECTROSTATIC IMAGE ON A PHOTOCONDUCING INSULATINGSURFACE, EVELOPING SAID FIRST LATENT ELECTROSTATIC IMAGE WITH AFINELY-DIVIDED DEVELOPER POWDER THEREBY FORMING A FIRST VISIBLE POWDERIMAGE ON SAID SURFACE, UNIFORMLY EXPOSING SAID SURFACE BEARING SAIDFIRST VISIBLE POWDER IMAGE TO LIGHT TO WHICH SAID SURFACE IS SENSITIVETO SUBSTANTIALLY DISCHARGE THE ELECTROSTATIC CHARGES REMAINING ON SAIDSURFACE AFTER PRODUCING SAID FIRST LATENT ELECTROSTATIC IMAGE WHILELEAVING SAID FIRST VISIBLE POWDER IMAGE SUBSTANTIALLY INDISTURBED ANDADHERING TO SAID SURFACE, EXPOSING SAID SURFACE BEARING SAID FIRSTVISIBLE POWDER IMAGE THEREON TO A CORONA DISCHARGE TO PRODUCE A SECONDLATENT ELECTROSTATIC IMAGE THEREON IN REVERSE CONFIGURATION WITH RESPECTTO SAID FIRST LATENT ELECTROSTATIC IMAGE BUT HAVING THE SAME POLARITY OFCHANGE AS SAID FIRST LATENT ELECTROSTATIC IMAGE, REMOV-